1. Field of the Invention
This invention relates to a thermographic recording element and more particularly, to a photothermographic recording element suited for the manufacture of graphic printing plates.
2. Prior Art
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d in xe2x80x9cImaging Processes and Materials,xe2x80x9d Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tone of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80xc2x0 C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Such photothermographic materials have been used as microphotographic and radiographic photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices. The demand for simple dry processing is also increasing.
U.S. Pat. No. 3,667,958 discloses that a photothermographic element comprising a polyhydroxybenzene combined with a hydroxylamine, reductone or hydrazine has high image quality discrimination and resolution. This combination of reducing agents, however, was found to incur an increase of fog.
U.S. Pat. No. 5,496,695 discloses a heat-developable photothermographic element comprising an organic silver salt, a silver halide, a hindered phenol, and a certain hydrazine derivative. These hydrazine derivatives were found still insufficient to accomplish a maximum ultimate density or ultrahigh contrast.
U.S. Pat. No. 5,545,515 discloses the use of acrylonitriles as the co-developer. The hydrazine compounds used therein fail to achieve a fully satisfactory high contrast while the occurrence of black peppers was ascertained.
An object of the present invention is to provide a thermographic recording element having a high sensitivity and high Dmax and free of black peppers. Another object of the present invention is to provide a printing plate-forming photosensitive element which can be processed in a fully dry basis without a need for wet processing and produce images of quality.
According to the invention, there is provided a thermographic recording element having at least one image forming layer. The element contains an organic silver salt, a reducing agent, and at least one of substituted alkene derivatives of the general formulae (1) through (14). 
In formulae (1) through (14), W is an electron attractive group, D is an electron donative group, and H is hydrogen. The groups represented by W or D attached to the same carbon atom, taken together, may form a cyclic structure. The compound may assume either a trans or a cis structure when both trans and cis structures are possible with respect to W or D. Two W groups in formula (14) form a cyclic structure.
In one preferred embodiment, the thermographic recording element further contains a hydrazine derivative of the general formula (I). 
In formula (I), R2 is an aliphatic, aromatic or heterocyclic group, R1 is hydrogen or a block group, G1 is xe2x80x94COxe2x80x94, xe2x80x94COCOxe2x80x94, xe2x80x94C(xe2x95x90S)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94PO(R3)xe2x80x94 or iminomethylene group, R3 is selected from the same range as defined for R1 and may be different from R1, A1 and A2 are independently hydrogen, alkylsulfonyl, arylsulfonyl or acyl groups, at least one of A1 and A2 is hydrogen, and letter m1 is equal to 0 or 1, with the proviso that R1 is an aliphatic, aromatic or heterocyclic group when m1 is 0.
In one preferred embodiment, the thermographic recording element further contains a photosensitive silver halide so that the element may be photosensitive. That is, a photothermographic (or photosensitive, heat-developable) recording element is provided.
The thermographic (or heat-developable) recording element of the invention has at least one image forming layer and contains an organic silver salt and a reducing agent. Preferably it further contains a photosensitive silver halide whereby the invention constitutes a photo-thermographic (or photosensitive, heat-developable) recording element. According to the feature of the Invention, the element further contains substituted alkene derivatives of the general formulae (1) through (14). The inclusion of such substituted alkene derivatives not only provides the thermographic recording element with a high Dmax, high sensitivity, and fully high contrast, but is also effective for suppressing the occurrence of black peppers.
These advantages are enhanced by further adding a hydrazine derivative of the general formula (I).
First, the substituted alkene derivatives of the general formulae (1) through (14) are described in detail.
In formulae (1) through (14), W is an electron attractive group, D is an electron donative group, and H is a hydrogen atom. The groups represented by W or D attached to the same carbon atom, taken together, may form a cyclic structure. When both trans and cis structures are possible with respect to W or D, the compound may assume either a trans or a cis structure. Two W groups in formula (14) form a cyclic structure.
In formulae (1) through (14), the electron attractive groups represented by W include halogen atoms, cyano groups, nitro groups, alkenyl groups, alkynyl groups, acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, alkylsulfonyl groups, arylsulfonyl groups, carbamoyl groups, carbonamide groups, sulfamoyl groups, sulfonamide groups, trifluoromethyl groups, trichloromethyl groups, phosphoryl groups, carboxy groups (or salts thereof), sulfo groups (or salts thereof), heterocyclic groups, imino groups, and phenyl groups having such electron attractive groups as a substituent. These groups may have substituents, examples of which include halogen atoms (e.g., fluorine, chlorine, bromine and iodine atoms), alkyl groups (including aralkyl, cycloalkyl and active methine groups), alkenyl groups, alkynyl groups, aryl groups, heterocyclic groups, quaternized nitrogen atom-containing heterocyclic groups (such as pyridinio), acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, carboxy groups or salts thereof, sulfonylcarbamoyl groups, acylcarbamoyl groups, sulfamoylcarbamoyl groups, carbazoyl groups, oxalyl groups, oxamoyl groups, cyano groups, thiocarbamoyl groups, hydroxy groups, alkoxy groups (including groups containing recurring ethyleneoxy or propyleneoxy units), aryloxy groups, heterocyclic oxy groups, acyloxy groups, (alkoxy or aryloxy) carbonyloxy groups, carbamoyloxy groups, sulfonyloxy groups, amino groups, (alkyl, aryl or heterocyclic) amino groups, N-substituted nitrogenous heterocyclic groups, acylamino groups, sulfonamide groups, ureido groups, thioureido groups, imide groups, (alkoxy or aryloxy) carbonylamino groups, sulfamoylamino groups, semicarbazide groups, thiosemicarbazide groups, hydrazino groups, quaternary ammonio groups, oxamoylamino groups, (alkyl or aryl) sulfonylureido groups, acylureido groups, acylsulfamoylamino groups, nitro groups, mercapto groups, (alkyl, aryl or heterocyclic) thio groups, (alkyl or aryl) sulfonyl groups, (alkyl or aryl) sulfinyl groups, sulfo groups or salts thereof, sulfamoyl groups, acylsulfamoyl groups, sulfonylsulfamoyl groups or salts thereof, and phosphoramide or phosphate structure-bearing groups. These substituents may be further replaced by other substituents selected from the foregoing examples.
The preferred electron attractive groups are those having 0 to 16 carbon atoms in total, especially 0 to 12 carbon atoms in total, for example, cyano, nitro, alkenyl, acyl, alkoxycarbonyl, aryloxycarbonyl, alkylsulfonyl, arylsulfonyl, carbamoyl, sulfamoyl, trifluoromethyl, phosphoryl groups, heterocyclic groups (5- and 6-membered heterocyclic groups which may have a benzene or naphthalene ring fused thereto), and phenyl groups having any electron attractive group as a substituent. It is noted that the heterocyclic groups in formulae (3) and (7) are non-aromatic heterocyclic groups.
In formulae (1) through (14), the electron donative groups represented by D include hydroxy groups (or salts thereof), mercapto groups (or salts thereof), alkoxy groups, aryloxy groups, heterocyclic oxy groups, alkylthio groups, arylthio groups, heterocyclic thio groups, amino groups, alkylamino groups, arylamino groups, heterocyclic amino groups, and phenyl groups having such electron donative groups as a substituent. These groups may have substituents, examples of which are the same as described for W.
The preferred electron donative groups are hydroxy groups (or salts thereof), mercapto groups (or salts thereof), alkoxy groups, alkylthio groups, arylthio groups, amino groups, alkylamino groups, arylamino groups, and phenyl groups having any electron donative group as a substituent.
Examples of the ring formed by W and D include saturated or unsaturated, carbocyclic or heterocyclic rings which may have a 4- to 6-membered ring fused thereto. Also the ring may be a cyclic ketone. The heterocyclic ring preferably contains at least one atom of nitrogen, oxygen and sulfur, more preferably one or two such hetero atoms.
Preferred among the compounds of formulae (1) through (14) are those of formulae (1), (2), (3), (5), (7), (8), (10), (11), (12), (13), and (14). More preferred are the compounds of formulae (1), (2), (3), (5), (7), (8), (10), (11), (12), and (14).
Illustrative, non-limiting, examples of the compounds of formulae (1) through (14) are given below. 
The compounds according to the invention can be readily synthesized by well-known methods and are also commercially available from chemical manufacturers.
These compounds may be used alone or in admixture of two or more. The amount of the compound(s) added is preferably 1xc3x9710xe2x88x926 to 1 mol, more preferably 1xc3x9710xe2x88x925 to 5xc3x9710xe2x88x921 mol, and most preferably 2xc3x9710xe2x88x925 to 2xc3x9710xe2x88x921 mol per mol of silver. The compound may be added to an image forming layer or any other layer on the image forming layer side of a support, and preferably to the image forming layer or a layer disposed adjacent thereto.
In the practice of the invention, the substituted alkene derivative is used as solution in water or a suitable organic solvent. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethylsulfoxide and methyl cellosolve.
A well-known emulsifying dispersion method is used for dissolving the substituted alkene derivative with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the substituted alkene derivative in powder form in water in a ball mill, colloidal mill or ultrasonic mixer.
In the preferred embodiment, the thermographic recording element of the invention contains a hydrazine derivative of the general formula (I). 
Herein R2 is an aliphatic, aromatic or heterocyclic group. R1 is hydrogen or a block group. G1 is xe2x80x94COxe2x80x94, xe2x80x94COCOxe2x80x94, xe2x80x94C(xe2x95x90S)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94PO(R3)xe2x80x94 or iminomethylene group. R3 is selected from the same range as defined for R1 and may be different from R1. A1 and A2 are both hydrogen, or one of A1 and A2 is hydrogen and the other is a substituted or unsubstituted alkylsulfonyl, substituted or unsubstituted arylsulfonyl or substituted or unsubstituted acyl group. Letter m1 is equal to 0 or 1. R1 is an aliphatic, aromatic or heterocyclic group when m1 is 0.
In formula (I), the aliphatic groups represented by R2 are preferably substituted or unsubstituted, normal, branched or cyclic alkyl, alkenyl and alkynyl groups having 1 to 30 carbon atoms.
In formula (I), the aromatic groups represented by R2 are preferably monocyclic or fused ring aryl groups, for example, phenyl and naphthyl groups derived from benzene and naphthalene rings. The heterocyclic groups represented by R2 are preferably monocyclic or fused ring, saturated or unsaturated, aromatic or non-aromatic heterocyclic groups while the heterocycles in these groups include pyridine, pyrimidine, imidazole, pyrazole, quinoline, isoquinoline, benzimidazole, thiazole, benzothiazole, piperidine, triazine, morpholine, and piperazine rings.
Aryl, alkyl and aromatic heterocyclic groups are most preferred as R2.
The group represented by R2 may have a substituent. Exemplary substituents include halogen atoms (e.g., fluorine, chlorine, bromine and iodine), alkyl groups (inclusive of aralkyl, cycloalkyl and active methine groups), alkenyl groups, alkynyl groups, aryl groups, heterocyclic groups, heterocyclic groups containing a quaternized nitrogen atom (e.g., pyridinio), acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, carboxy groups or salts thereof, sulfonylcarbamoyl groups, acylcarbamoyl groups, sulfamoylcarbamoyl groups, carbazoyl groups, oxalyl groups, oxamoyl groups, cyano groups, thiocarbamoyl groups, hydroxy groups, alkoxy groups (inclusive of groups having recurring ethylenoxy or propylenoxy units), aryloxy groups, heterocyclic oxy groups, acyloxy groups, (alkoxy or aryloxy)carbonyloxy groups, carbamoyloxy groups, sulfonyloxy groups, amino groups, (alkyl, aryl or heterocyclic) amino groups, N-substituted nitrogenous heterocyclic groups, acylamino groups, sulfon-amide groups, ureido groups, thioureido groups, imide groups, (alkoxy or aryloxy)carbonylamino groups, sulfamoyl-amino groups, semicarbazide groups, thiosemicarbazide groups, hydrazino groups, quaternary ammonio groups, oxamoylamino groups, (alkyl or aryl)sulfonylureido groups, acylureido groups, acylsulfamoylamino groups, nitro groups, mercapto groups, (alkyl, aryl or heterocyclic) thio groups, (alkyl or aryl)sulfonyl groups, (alkyl or aryl)sulfinyl groups, sulfo groups or salts thereof, sulfamoyl groups, acylsulfamoyl groups, sulfonylsulfamoyl groups or salts thereof, and groups containing a phosphoramide or phosphoric ester structure. These substituents may be further substituted with such a substituent.
Preferred substituents that R2 may have include, where R2 is an aromatic or heterocyclic group, alkyl (inclusive of active methylene), aralkyl, heterocyclic, substituted amino, acylamino, sulfonamide, ureido, sulfamoylamino, imide, thioureido, phosphoramide, hydroxy, alkoxy, aryloxy, acyloxy, acyl, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, carboxy (inclusive of salts thereof), (alkyl, aryl or heterocyclic) thio, sulfo (inclusive of salts thereof), sulfamoyl, halogen, cyano, and nitro groups.
Where R2 is an aliphatic group, preferred substituents include alkyl, aryl, heterocyclic, amino, acylamino, sulfonamide, ureido, sulfamoylamino, imide, thioureido, phosphoramide, hydroxy, alkoxy, aryloxy, acyloxy, acyl, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, carboxy (inclusive of salts thereof), (alkyl, aryl or heterocyclic) thio, sulfo (inclusive of salts thereof), sulfamoyl, halogen, cyano, and nitro groups.
In formula (I), R1 is hydrogen or a block group. Examples of the block group include aliphatic groups (e.g., alkyl, alkenyl and alkynyl groups), aromatic groups (monocyclic or fused ring aryl groups), heterocyclic groups, alkoxy, aryloxy, amino and hydrazino groups.
The alkyl groups represented by R1 are preferably substituted or unsubstituted alkyl groups having 1 to 10 carbon atoms, for example, methyl, ethyl, trifluoromethyl, difluoromethyl, 2-carboxytetrafluoroethyl, pyridiniomethyl, difluoromethoxymethyl, difluorocarboxymethyl, 3-hydroxy-propyl, hydroxymethyl, 3-methanesulfonamidopropyl, benzenesulfonamidomethyl, trifluoroacetylmethyl, dimethyl-aminomethyl, phenylsulfonylmethyl, o-hydroxybenzyl, methoxymethyl, phenoxymethyl, 4-ethylphenoxymethyl, phenylthiomethyl, t-butyl, dicyanomethyl, diphenylmethyl, triphenylmethyl, methoxycarbonyldiphenylmethyl, cyano-diphenylmethyl, and methylthiodiphenylmethyl groups. The alkenyl groups are preferably those having 1 to 10 carbon atoms, for example, vinyl, 2-ethoxycarbonylvinyl, and 2-trifluoro-2-methoxycarbonylvinyl groups. The alkynyl groups are preferably those having 1 to 10 carbon atoms, for example, ethynyl and 2-methoxycarbonylethynyl groups. The aryl groups are preferably monocyclic or fused ring aryl groups, especially those containing a benzene ring, for example, phenyl, perfluorophenyl, 3,5-dichlorophenyl, 2-methanesulfonamidophenyl, 2-carbamoylphenyl, 4,5-dicyano-phenyl, 2-hydroxymethylphenyl, 2,6-dichloro-4-cyanophenyl, and 2-chloro-5-octylsulfamoylphenyl groups.
The heterocyclic groups represented by R1 are preferably 5- and 6-membered, saturated or unsaturated, monocyclic or fused ring, heterocyclic groups containing at least one of nitrogen, oxygen and sulfur atoms, for example, morpholino, piperidino (N-substituted), imidazolyl, indazolyl (e.g., 4-nitroindazolyl), pyrazolyl, triazolyl, benzimidazolyl, tetrazolyl, pyridyl, pyridinio (e.g., N-methyl-3-pyridinio), quinolinio, quinolyl, hydantoyl and imidazolidinyl groups.
The alkoxy groups are preferably those having 1 to 8 carbon atoms, for example, methoxy, 2-hydroxyethoxy, benzyloxy, and t-butoxy groups. The aryloxy groups are preferably substituted or unsubstituted phenoxy groups. The amino groups are preferably unsubstituted amino, alkylamino having 1 to 10 carbon atoms, arylamino, and saturated or unsaturated heterocyclic amino groups (inclusive of nitrogenous heterocyclic amino groups containing a quaternized nitrogen atom). Examples of the amino group include 2,2,6,6-tetramethylpiperidin-4-ylamino, propylamino, 2-hydroxyethylamino, anilino, o-hydroxyanilino, 5-benzo-triazolylamino, and N-benzyl-3-pyridinioamino groups. The hydrazino groups are preferably substituted or unsubstituted hydrazino groups and substituted or unsubstituted phenylhydrazino groups (e.g., 4-benzenesulfonamidophenyl-hydrazino).
The groups represented by R1 may be substituted ones, with examples of the substituent being as exemplified for the substituent on R2.
In formula (I), R1 may be such a group as to induce cyclization reaction to cleave a G1xe2x80x94R1 moiety from the remaining molecule to generate a cyclic structure containing the atoms of the xe2x80x94G1xe2x80x94R1 moiety. Such examples are described in JP-A 29751/1988, for example.
The hydrazine derivative of formula (I) may have incorporated therein a group capable of adsorbing to silver halide. Such adsorptive groups include alkylthio, arylthio, thiourea, thioamide, mercapto heterocyclic and triazole groups as described in U.S. Pat. Nos. 4,385,108 and 4,459,347, JP-A 195233/1984, 200231/1984, 201045/1984, 201046/1984, 201047/1984, 201048/1984, 201049/1984, 170733/1986, 270744/1986, 948/1987, 234244/1988, 234245/1988, and 234246/1988. These adsorptive groups to silver halide may take the form of precursors. Such precursors are exemplified by the groups described in JP-A 285344/1990.
R1 and R2 in formula (I) may have incorporated therein a ballast group or polymer commonly used in immobile photographic additives such as couplers. The ballast group is a group having at least 8 carbon atoms and relatively inert with respect to photographic properties. It may be selected from, for example, alkyl, aralkyl, alkoxy, phenyl, alkylphenyl, phenoxy, and alkylphenoxy groups. The polymer is exemplified in JP-A 100530/1989, for example.
R1 or R2 in formula (I) may have a plurality of hydrazino groups as a substituent. In this case, the compounds of formula (I) are polymeric with respect to hydrazino group. Exemplary polymeric compounds are described in JP-A 86134/1989, 16938/1992, 197091/1993, WO 95-32452 and 95-32453, Japanese Patent Application Nos. 351132/1995, 351269/1995, 351168/1995, 351287/1995, and 351279/1995.
R1 or R2 in formula (I) may contain a cationic group (e.g., a group containing a quaternary ammonio group and a nitrogenous heterocyclic group containing a quaternized nitrogen atom), a group containing recurring ethyleneoxy or propyleneoxy units, an (alkyl, aryl or heterocyclic) thio group, or a group which is dissociable with a base (e.g., carboxy, sulfo, acylsulfamoyl, and carbamoylsulfamoyl). Exemplary compounds containing such a group are described in, for example, in JP-A 234471/1995, 333466/1993, 19032/1994, 19031/1994, 45761/1993, 259240/1991, 5610/1995, and 244348/1995, U.S. Pat. Nos. 4,994,365 and 4,988,604, and German Patent No. 4006032.
In formula (I), each of A1 and A2 is a hydrogen atom, a substituted or unsubstituted alkyl- or arylsulfonyl group having up to 20 carbon atoms (preferably a phenylsulfonyl group or a phenylsulfonyl group substituted such that the sum of Hammette""s substituent constants may be xe2x88x920.5 or more), or a substituted or unsubstituted acyl group having up to 20 carbon atoms (preferably a benzoyl group, a benzoyl group substituted such that the sum of Hammette""s substituent constants may be xe2x88x920.5 or more, or a linear, branched or cyclic, substituted or unsubstituted, aliphatic acyl group wherein the substituent is selected from a halogen atom, ether group, sulfonamide group, carbonamide group, hydroxyl group, carboxy group and sulfo group). Most preferably, both A1 and A2 are hydrogen atoms.
The preferable range of the hydrazine derivatives of the general formula (I) is described.
In formula (I), R2 is preferably phenyl, substituted alkyl of 1 to 3 carbon atoms or aromatic heterocyclic groups.
Where R2 represents phenyl or aromatic heterocyclic groups, preferred substituents thereon include nitro, cyano, alkoxy, alkyl, acylamino, ureido, sulfonamide, thioureido, carbamoyl, sulfamoyl, sulfonyl, carboxy (or salts thereof), sulfo (or salts thereof), alkoxycarbonyl, and chloro groups.
Where R2 represents substituted alkyl groups of 1 to 3 carbon atoms, it is more preferably substituted methyl groups, and further preferably di- or tri-substituted methyl groups. Exemplary preferred substituents on these methyl groups include methyl, phenyl, cyano, (alkyl, aryl or heterocyclic) thio, alkoxy, aryloxy, chloro, heterocyclic, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, sulfamoyl, amino, acylamino, and sulfonamide groups, and especially, substituted or unsubstituted phenyl groups.
Where R2 represents substituted methyl groups, preferred examples thereof are t-butyl, dicyanomethyl, dicyanophenylmethyl, triphenylmethyl (trityl), diphenyl-methyl, methoxycarbonyldiphenylmethyl, cyanodiphenylmethyl, methylthiodiphenylmethyl, cyclopropyldiphenylmethyl groups, with trityl being most preferred.
Where R2 represents heterocyclic groups, preferred examples thereof are pyridine, quinoline, pyrimidine, triazine, benzthiazole, benzimidazole, and thiophene rings.
Most preferably, R2 in formula (I) represents substituted phenyl groups.
In formula (I), m1 is equal to 0 or 1. When m1 is 0, R1 represents aliphatic, aromatic or heterocyclic groups. When m1 is 0, R1 more preferably represents phenyl groups, substituted alkyl groups of 1 to 3 carbon atoms or alkenyl groups. Of these, the preferred ranges of the phenyl groups and the substituted alkyl groups of 1 to 3 carbon atoms are the same as the preferred range of R2. Where R1 represents alkenyl groups, they are preferably vinyl groups, more preferably vinyl groups having one or two substituents selected from cyano, acyl, alkoxycarbonyl, nitro, trifluoromethyl and carbamoyl groups. Exemplary are 2,2-dicyanovinyl, 2-cyano-2-methoxycarbonylvinyl, and 2-acetyl-2-ethoxycarbonylvinyl groups.
Preferably m1 is equal to 1.
Where R2 is a phenyl or aromatic heterocyclic group and G1 is xe2x80x94COxe2x80x94, the groups represented by R1 are preferably selected from hydrogen, alkyl, alkenyl, alkynyl, aryl and heterocyclic groups, more preferably from hydrogen, alkyl and aryl groups, and most preferably from hydrogen atoms and alkyl groups. Where R1 represents alkyl groups, preferred substituents thereon are halogen, alkoxy, aryloxy, alkylthio, arylthio, hydroxy, sulfonamide, amino, acylamino, and carboxy groups.
Where R2 is a substituted methyl group and G1 is xe2x80x94COxe2x80x94, the groups represented by R1 are preferably selected from hydrogen, alkyl, aryl, heterocyclic, alkoxy, and amino groups (including unsubstituted amino, alkylamino, arylamino and heterocyclic amino groups), more preferably from hydrogen, alkyl, aryl, heterocyclic, alkoxy, alkylamino, arylamio and heterocyclic amino groups. Where G1 is xe2x80x94COCOxe2x80x94, independent of R2, R1 is preferably selected from alkoxy, aryloxy, and amino groups, more preferably from substituted amino groups, specifically alkylamino, arylamino and saturated or unsaturated heterocyclic amino groups.
Where G1 is xe2x80x94SO2xe2x80x94, independent of R2, R1 is preferably selected from alkyl, aryl and substituted amino groups.
In formula (I), G1 is preferably xe2x80x94COxe2x80x94 or xe2x80x94COCOxe2x80x94, and most preferably xe2x80x94COxe2x80x94.
Illustrative, non-limiting, examples of the compound represented by formula (I) are given below.
The compounds of formula (I) may be used alone or in admixture of two or more.
In addition to the above-described ones, the following hydrazine derivatives are also preferable for use in the practice of the invention. If desired, any of the following hydrazine derivatives may be used in combination with the hydrazine derivatives of formula (I). The hydrazine derivatives which are used herein can be synthesized by various methods as described in the following patents.
Exemplary hydrazine derivatives which can be used herein include the compounds of the chemical formula [1] in JP-B 77138/1994, more specifically the compounds described on pages 3 and 4 of the same; the compounds of the general formula (I) in JP-B 93082/1994, more specifically compound Nos. 1 to 38 described on pages 8 to 18 of the same; the compounds of the general formulae (4), (5) and (6) in JP-A 230497/1994, more specifically compounds 4-1 to 4-10 described on pages 25 and 26, compounds 5-1 to 5-42 described on pages 28 to 36, and compounds 6-1 to 6-7 described on pages 39 and 40 of the same; the compounds of the general formulae (1) and (2) in JP-A 289520/1994, more specifically compounds 1-1 to 1-17 and 2-1 described on pages 5 to 7 of the same; the compounds of the chemical formulae [2] and [3] in JP-A 313936/1994, more specifically the compounds described on pages 6 to 19 of the same; the compounds of the chemical formula [1] in JP-A 313951/1994, more specifically the compounds described on pages 3 to 5 of the same; the compounds of the general formula (I) in JP-A 5610/1995, more specifically compounds I-1 to I-38 described on pages 5 to 10 of the same; the compounds of the general formula (II) in JP-A 77783/1995, more specifically compounds II-1 to II-102 described on pages 10 to 27 of the same; the compounds of the general formulae (H) and (Ha) in JP-A 104426/1995, more specifically compounds H-1 to H-44 described on pages 8 to 15 of the same; the compounds having an anionic group in proximity to a hydrazine group or a nonionic group capable of forming an intramolecular hydrogen bond with the hydrogen atom of hydrazine described in EP 713131A, especially compounds of the general formulae (A), (B), (C), (D), (E), and (F), more specifically compounds N-1 to N-30 described therein; and the compounds of the general formula (1) in EP 713131A, more specifically compounds D-1 to D-55 described therein.
Also useful are the hydrazine derivatives described in xe2x80x9cKnown Technology,xe2x80x9d Aztech K.K., Mar. 22, 1991, pages 25-34 and Compounds D-2 and D-39 described in JP-A 86354/1987, pages 6-7.
In the practice of the invention, the hydrazine nucleating agent is used as solution in water or a suitable organic solvent. Suitable solvents include alcohols (e.g., methanol, ethanol, propanol, and fluorinated alcohols), ketones (e.g., acetone and methyl ethyl ketone), dimethylformamide, dimethylsulfoxide and methyl cellosolve.
A well-known emulsifying dispersion method is used for dissolving the hydrazine derivative with the aid of an oil such as dibutyl phthalate, tricresyl phosphate, glyceryl triacetate or diethyl phthalate or an auxiliary solvent such as ethyl acetate or cyclohexanone whereby an emulsified dispersion is mechanically prepared. Alternatively, a method known as a solid dispersion method is used for dispersing the hydrazine derivative in powder form in water in a ball mill, colloidal mill or ultrasonic mixer.
The hydrazine nucleating agent may be added to an image forming layer or any other layer on the image forming layer side of a support, and preferably to the image forming layer or a layer disposed adjacent thereto.
The hydrazine nucleating agent is preferably used in an amount of 1xc3x9710xe2x88x926 mol to 1 mol, more preferably 1xc3x97105 mol to 5xc3x9710xe2x88x921 mol, and most preferably 2xc3x9710xe2x88x925 mol to 2xc3x9710xe2x88x921 mol per mol of silver.
In the thermographic recording element of the invention, a nucleation promoter may be added in combination with the hydrazine derivative. The nucleation promoter used herein includes amine derivatives, onium salts, disulfide derivatives, and hydroxylamine derivatives.
Examples of the nucleation promoter are shown below. 
Other useful examples of the nucleation promoter include the compounds described in JP-A 77783/1995, page 48, lines 2-37, more specifically Compounds A-1 to A-73 described on pages 49-58 of the same; the compounds of the chemical formulae [21], [22] and [23] described in JP-A 84331/1995, more specifically the compounds described on pages 6-8 of the same; the compounds of the general formulae [Na] and [Nb] described in JP-A 104426/1995, more specifically Compounds Na-1 to Na-22 and Nb-1 to Nb-12 described on pages 16-20 of the same; the compounds of the general formulae (1), (2), (3), (4), (5), (6) and (7) described in JP-A 37817/1995, more specifically Compounds 1-1 to 1-19, Compounds 2-1 to 2-22, Compounds 3-1 to 3-36, Compounds 4-1 to 4-5, Compounds 5-1 to 5-41, Compounds 6-1 to 6-58 and Compounds 7-1 to 7-38 described therein; and the nucleation promoters described in Japanese Patent Application No. 70908/1996.
The nucleation promoter is preferably used in an amount of 1xc3x9710xe2x88x926 mol to 2xc3x9710xe2x88x922 mol, more preferably 1xc3x9710xe2x88x925 mol to 2xc3x9710xe2x88x922 mol, and most preferably 2xc3x9710xe2x88x925 to 1xc3x9710xe2x88x922 mol per mol of silver.
The thermographic recording element of the invention contains a reducing agent. In one embodiment of the invention, a compound of the following general formula (A) is contained as the reducing agent in the thermographic recording element. 
Herein, R is hydrogen or alkyl groups having 1 to 10 carbon atoms such as xe2x80x94C4H9 and 2,4,4-trimethylpentyl, Rxe2x80x2 and Rxe2x80x3 are alkyl groups having 1 to 5 carbon atoms such as methyl, ethyl and n-butyl.
Illustrative, non-limiting examples of the compound of formula (A) are given below. 
The compound of formula (A) is preferably used in an amount of 1xc3x9710xe2x88x922 to 10 mol, more preferably 1xc3x9710xe2x88x922 to 1.5 mol per mol of silver.
In another embodiment of the invention, compounds of the following general formulae (R-I), (R-II), (R-III) and (R-IV) are contained as the reducing agent in the thermographic recording element. 
In formula (R-III), Z forms a cyclic structure represented by the following formula (Z-1) or (Z-2). 
In formula (R-IV), Z forms a cyclic structure represented by the following formula (Z-3) or (Z-4). 
In formulae (R-I) and (R-II), each of L1 and L2 is a group xe2x80x94CHxe2x80x94(R6)xe2x80x94, xe2x80x94CHxe2x80x94(R6xe2x80x2)xe2x80x94 or a sulfur atom, and n is a natural number.
Herein, R1 to R5, R6, R7 to R10, R1xe2x80x2 to R5, R6xe2x80x2, R11 to R13, R11xe2x80x2 to R13xe2x80x2, R21 to R26, R21xe2x80x2 to R24xe2x80x2 are hydrogen atoms, alkyl groups, aryl groups, aralkyl groups, halogen atoms, amino groups or substituents represented by xe2x80x94OA, with the proviso that at least one of R1 to R5, at least one of R1xe2x80x2 to R5xe2x80x2, and at least one of R7 to R10 each are a group represented by xe2x80x94OA. In formula (R-II), L1 is a sulfur atom where at least one of R1 to R5 and at least one of R1xe2x80x2 to R5xe2x80x2 are groups represented by xe2x80x94OA. Also, a plurality of substituents in each of R1 to R5, R7 to R10, R1xe2x80x2 to R5xe2x80x2, R11 to R13, R11xe2x80x2 to R13xe2x80x2, R21 to R26, R21xe2x80x2 to R24xe2x80x2, taken together, may form a ring.
A and Axe2x80x2 each are a hydrogen atom, alkyl group having 1 to 30 carbon atoms, acyl group having 1 to 30 carbon atoms, aryl group, phosphate group or sulfonyl group.
R1 to R5, R7 to R10, R1xe2x80x2 to R5xe2x80x2, R11 to R13, R11xe2x80x2 to R13xe2x80x2, R21 to R26, R21xe2x80x2 to R24xe2x80x2, A and Axe2x80x2 may be substituted groups while typical examples of the substituent include alkyl groups (including active methine groups), nitro groups, alkenyl groups, alkynyl groups, aryl groups, heterocyclic ring-containing groups, groups containing a quaternized nitrogen atom-containing heterocyclic ring (e.g., pyridinio group), hydroxy groups, alkoxy groups (including groups containing recurring ethyleneoxy or propyleneoxy units), aryloxy groups, acyloxy groups, acyl groups, alkoxycarbonyl groups, aryloxycarbonyl groups, carbamoyl groups, urethane groups, carboxyl groups, imido groups, amino groups, carbonamide groups, sulfonamide groups, ureido groups, thioureido groups, sulfamoylamino groups, semicarbazide groups, thiosemicarbazide groups, hydrazino-containing groups, quaternary ammonio-containing groups, mercapto groups, (alkyl, aryl or heterocyclic) thio groups, (alkyl or aryl) sulfonyl groups, (alkyl or aryl) sulfinyl groups, sulfo groups, sulfamoyl groups, acylsulfamoyl groups, (alkyl or aryl) sulfonylureido groups, (alkyl or aryl) sulfonylcarbamoyl groups, halogen atoms, cyano groups, phosphoramide groups, phosphate structure-containing groups, acylurea structure-bearing groups, selenium or tellurium atom-containing groups, and tertiary or quaternary sulfonium structure-bearing groups. These substituents may be further substituted, with preferred examples of the further substituent being the same as the foregoing substituents.
Illustrative, non-limiting, examples of the compounds represented by formulae (R-I), (R-II), (R-III) and (R-IV) are given below.
TMB: 1,3,3-trimethylbutyl group
CPen: cyclopentyl group
CHex: cyclohexyl group
The reducing agents represented by the general formulae (R-I) to (R-IV) are preferably used in amounts of 1xc3x9710xe2x88x923 to 10 mol, more preferably 1xc3x9710xe2x88x922 to 1.5 mol per mol of silver.
The compounds of formula (A) and the compounds of formulae (R-I) to (R-IV) may be used alone or in admixture. When they are used in admixture, the auxiliary reducing agent and the main reducing agent are preferably used in a molar ratio between 1/1000 and 1/1, more preferably between 1/100 and 1/1.
The thermographic recording element according to the invention is processed by a heat development process to form photographic images. As described in the preamble, photothermographic elements which constitute one embodiment of the invention are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d in xe2x80x9cImaging Processes and Materials,xe2x80x9d Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
The thermographic recording element of the invention which forms photographic images through heat development preferably contains a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tone of silver, and a reducing agent, typically dispersed in a binder (typically organic binder) matrix. Although the photothermographic material is stable at room temperature, it is developed merely by heating at an elevated temperature (e.g., 80xc2x0 C. or higher) after exposure, that is, without a need for a processing solution. Upon heating, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
The thermographic recording element of the invention has at least one image forming layer, typically a photo-sensitive layer, on a support. It is acceptable to form only an image forming layer such as a photosensitive layer on a support although it is preferred to form at least one image protective layer such as a non-photosensitive layer on the image forming layer such as the photosensitive layer.
In the photothermographic elements which constitute the preferred embodiment of the invention, in order to control the quantity or wavelength distribution of light transmitted to the photosensitive layer, a filter layer may be formed on the same side as or on the opposite side to the photo-sensitive layer, or a dyestuff or pigment may be contained in the photosensitive layer. The dyestuff used to this end is preferably selected from the compounds described in Japanese Patent Application No. 11184/1995.
The photosensitive layer serving as the image forming layer may consist of two or more strata. Also a combination of high/low sensitivity strata or low/high sensitivity strata may be used for the adjustment of gradation.
Various additives may be added to any of the image forming layer (typically photosensitive layer), non-image-forming layer (typically non-photosensitive layer), and other layers.
In the thermographic recording element of the invention, various additives such as surfactants, antioxidants, stabilizers, plasticizers, UV absorbers, and coating aids may be used.
A binder is used to hold such additives. It is preferably transparent or semi-transparent and generally colorless. Exemplary binders are naturally occurring polymers, synthetic resins, polymers and copolymers, and other film-forming media, for example, gelatin, gum arabic, poly(vinyl alcohol), hydroxyethyl cellulose, cellulose acetate, cellulose acetate butyrate, polylvinyl pyrrolidone), casein, starch, poly(acrylic acid), poly(methyl methacrylate), polyvinyl chloride, poly(methacrylic acid), copoly(styrene-maleic anhydride), copoly(styrene-acrylonitrile), copoly(styrene-butadiene), poly(vinyl acetals) (e.g., poly(vinyl formal) and poly(vinyl butyral)), polyesters, polyurethanes, phenoxy resins, poly(vinylidene chloride), polyepoxides, polycarbonates, poly(vinyl acetate), cellulose esters, and polyamides. The binder may be dispersed in water, organic solvent or emulsion to form a dispersion which is coated to form a layer.
At least one layer of the image-forming layers used herein may be an image forming layer wherein a polymer latex constitutes more than 50% by weight of the entire binder. This image forming layer is sometimes referred to as xe2x80x9cinventive image-forming layerxe2x80x9d and the polymer latex used. as the binder therefor is referred to as xe2x80x9cinventive polymer latex,xe2x80x9d hereinafter. The term xe2x80x9cpolymer latexxe2x80x9d used herein is a dispersion of a microparticulate water-insoluble hydrophobic polymer in a water-soluble dispersing medium. With respect to the dispersed state, a polymer emulsified in a dispersing medium, an emulsion polymerized polymer, a micelle dispersion, and a polymer having a hydrophilic structure in a part of its molecule so that the molecular chain itself is dispersed on a molecular basis are included. With respect to the polymer latex, reference is made to Okuda and Inagaki Ed., xe2x80x9cSynthetic Resin Emulsion,xe2x80x9d Kobunshi Kankokai, 1978; Sugimura, Kataoka, Suzuki and Kasahara Ed., xe2x80x9cApplication of Synthetic Latex,xe2x80x9d Kobunshi Kankokai, 1993; and Muroi, xe2x80x9cChemistry of Synthetic Latex,xe2x80x9d Kobunshi Kankokai, 1970. Dispersed particles should preferably have a mean particle size of about 1 to 50,000 nm, more preferably about 5 to 1,000 nm. No particular limit is imposed on the particle size distribution of dispersed particles, and the dispersion may have either a wide particle size distribution or a monodisperse particle size distribution.
The inventive polymer latex used herein may be either a latex of the conventional uniform structure or a latex of the so-called core/shell type. In the latter case, better results are sometimes obtained when the core and the shell have different glass transition temperatures.
The inventive polymer latex should preferably have a minimum film-forming temperature (MFT) of about xe2x88x9230xc2x0 C. to 90xc2x0 C., more preferably about 0xc2x0 C. to 70xc2x0 C. A film-forming aid may be added in order to control the minimum film-forming temperature. The film-forming aid is also referred to as a plasticizer and includes organic compounds (typically organic solvents) for lowering the minimum film-forming temperature of a polymer latex. It is described in Muroi, xe2x80x9cChemistry of Synthetic Latex,xe2x80x9d Kobunshi Kankokai, 1970.
Polymers used in the inventive polymer latex according to the invention include acrylic resins, vinyl acetate resins, polyester resins, polyurethane resins, rubbery resins, vinyl chloride resins, vinylidene chloride resins, polyolefin resins, and copolymers thereof. The polymer may be linear or branched or crosslinked. The polymer may be either a homopolymer or a copolymer having two or more monomers polymerized together. The copolymer may be either a random copolymer or a block copolymer. The polymer preferably has a number average molecule weight Mn of about 5,000 to about 1,000,000, more preferably about 10,000 to about 100,000. Polymers with a too lower molecular weight would generally provide a low film strength after coating whereas polymers with a too higher molecular weight are difficult to form films.
The polymer of the inventive polymer latex should preferably have an equilibrium moisture content at 25xc2x0 C. and RH 60% of up to 2% by weight, more preferably up to 1% by weight. The lower limit of equilibrium moisture content is not critical although it is preferably 0.01% by weight, more preferably 0.03% by weight. With respect to the definition and measurement of equilibrium moisture content, reference should be made to xe2x80x9cPolymer Engineering Series No. 14, Polymer Material Test Methods,xe2x80x9d Edited by Japanese Polymer Society, Chijin Shokan Publishing K.K., for example.
Illustrative examples of the polymer latex which can be used as the binder in the image-forming layer of the thermo-graphic image recording element of the invention include latices of methyl methacrylate/ethyl acrylate/methacrylic acid copolymers, latices of methyl methacrylate/2-ethylhexyl acrylate/styrene/acrylic acid copolymers, latices of styrene/butadiene/acrylic acid copolymers, latices of styrene/butadiene/divinyl benzene/methacrylic acid copolymers, latices of methyl methacrylate/vinyl chloride/acrylic acid copolymers, and latices of vinylidene chloride/ethyl acrylate/acrylonitrile/methacrylic acid copolymers. These polymers or polymer latices are commercially available. Exemplary acrylic resins are Sebian A-4635, 46583 and 4601 (Daicell Chemical Industry K.K.) and Nipol LX811, 814, 820, 821 and 857 (Nippon Zeon K.K.). Exemplary polyester resins are FINETEX ES650, 611, 675, and 850 (Dai-Nippon Ink Chemical K.K.) and WD-size and WMS (Eastman Chemical Products, Inc.). Exemplary polyurethane resins are HYDRAN AP10, 20, 30 and 40 (Dai-Nippon Ink Chemical K.K.). Exemplary rubbery resins are LACSTAR 7310K, 3307B, 4700H and 7132C (Dai-Nippon Ink Chemical K.K.) and Nipol LX416, 410, 438C and 2507 (Nippon Zeon K.K.). Exemplary vinyl chloride resins are G351 and G576 (Nippon Zeon K.K.). Exemplary vinylidene chloride resins are L502 and L513 (Asahi Chemicals K.K.). Exemplary olefin resins are Chemipearl S120 and SA100 (Mitsui Petro-Chemical K.K.). These polymers may be used alone or in admixture of two or more.
In the inventive image-forming layer, the polymer latex described above is preferably used in an amount of at least 50% by weight, especially at least 70% by weight, of the entire binder. In the inventive image-forming layer, a hydrophilic polymer may be added in an amount of less than 50% by weight of the entire binder. Such hydrophilic polymers are gelatin, polyvinyl alcohol, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and hydroxypropyl methyl cellulose. The amount of the hydrophilic polymer added is preferably less than 30% by weight of the entire binder in the image-forming layer.
The inventive image-forming layer is preferably formed by applying an aqueous coating solution followed by drying. By the term xe2x80x9caqueousxe2x80x9d, it is meant that water accounts for at least 30% by weight of the solvent or dispersing medium of the coating solution. The component other than water of the coating solution may be a water-miscible organic solvent such as methyl alcohol, ethyl alcohol, isopropyl alcohol, methyl cellosolve, ethyl cellosolve, dimethylformamide or ethyl acetate. Exemplary solvent compositions include water, a 90/10 or 70/30 mixture of water/methanol, a 90/10 mixture of water/ethanol, a 90/10 mixture of water/isopropanol, a 95/5 mixture of water/dimethyl-formamide, a 80/15/5 or 90/5/5 mixture of water/methanol/dimethylformamide, all expressed in a weight ratio. The method described in U.S. Pat. No. 5,496,695 is also useful.
In the inventive image-forming layer, the total amount of binder is preferably 0.2 to 30 g/m2, more preferably 1 to 15 g/m2 per layer. To the image forming layer, crosslinking agents for crosslinking, surfactants for ease of application, and other addenda may be added.
Addition of toners is quite desirable. Preferred toners are disclosed in Research Disclosure No. 17029. Exemplary toners include imides such as phthalimide; cyclic imides such as succinimide, pyrazoline-5-ones, quinazolinone, 3-phenyl-2-pyrazoline-5-one, 1-phenylurazol, quinazoline and 2,4-thiazolizinedione; naphthalimides such as N-hydroxy-1,8-naphthalimide; cobalt complexes such as cobalt hexamine trifluoroacetate; mercaptans such as 3-mercapto-1,2,4-triazole; N-(aminomethyl)aryldicarboxyimides such as N-(dimethylaminomethyl)phthalimide; combinations of a blocked pyrazole, an isothiuronium derivative and a certain optical bleaching agent such as a combination of N,Nxe2x80x2-hexamethylenebis(1-carbamoyl-3,5-dimethylpyrazole), 1,8-(3,6-dioxaoctane)bis(isothiuroniumtrifluoroacetate) and 2-tribromomethylsulfonyl-benzothiazole; merocyanine dyes such as 3-ethyl-5-{(3-ethyl-2-benzothiazolinylidene)-1-methylethylidene}-2-thio-2,4-oxazolidinedione; phthalazinones, phthalazinone derivatives or metal salts thereof such as 4-(1-naphthyl)phthalazinone, 6-chloro-phthalazinone, 5,7-dimethyloxyphthalazinone and 2,3-dihydro-1,4-phthalazinedione; combinations of phthalazinones with sulfinic acid derivatives such as a combination of 6-chlorophthalazinone with sodium benzenesulfinate and a combination of 8-methylphthalazinone with sodium p-trisulfonate; combinations of phthalazines with phthalic acid; combinations of phthalazines (inclusive of phthalazine adducts) with maleic anhydride and at least one of phthalic acid, 2,3-naphthalenedicarboxylic acid and o-phenylenic acid derivative and anhydrides thereof (e.g., phthalic acid, 4-methylphthalic acid, 4-nitrophthalic acid, and tetrachloro-phthalic anhydride); quinazolinediones, benzoxazine, and naphthoxazine derivatives; benzoxazine-2,4-diones such as 1,3-benzoxazine-2,4-dione; pyrimidine and asym-triazines such as 2,4-dihydroxypyrimidine; and tetraazapentalene derivatives such as 3,6-dimercapto-1,4-diphenyl-1H,4H-2,3a,5,6a-tetraazapentalene. Phthalazones are preferred toners.
The silver halide which is useful as a catalytic amount of photocatalyst in the photothermographic element according to the preferred embodiment of the invention may be selected from photosensitive silver halides such as silver bromide, silver iodide, silver chloride, silver chlorobromide, silver iodobromide, and silver chloroiodobromide, with an iodide ion being preferably contained. The silver halide may be added to the image forming layer by any desired method whereupon the silver halide is disposed close to the reducible silver source. In general, the silver halide is preferably contained in an amount of 0.75 to 30% by weight based on the reducible silver source. The silver halide may be prepared by converting a silver soap moiety through reaction with a halide ion, or by preforming silver halide and adding it upon generation of a soap, or a combination of these methods. The latter method is preferred.
The reducible silver source is preferably selected from silver salts of organic and hetero-organic acids containing a reducible silver ion source, especially silver salts of long chain aliphatic carboxylic acids having 10 to 30 carbon atoms, especially 15 to 25 carbon atoms. Also preferred are complexes of organic or inorganic silver salts with ligands having an overall stability constant to silver ion in the range of 4.0 to 10.0. Preferred examples of the silver salt are described in Research Disclosure Nos. 17029 and 29963. Included are silver salts of organic acids (e.g., gallic acid, oxalic acid, behenic acid, stearic acid, palmitic acid, and lauric acid); silver salts of carboxyalkylthio-ureas (e.g., 1-(3-carboxypropyl)thiourea and 1-(3-carboxy-propyl)-3,3-dimethylthiourea); silver complexes of polymeric reaction products of aldehydes and hydroxy-substituted aromatic carboxylic acids (exemplary aldehydes are formaldehyde, acetaldehyde and butylaldehyde and exemplary hydroxy-substituted acids are salicylic acid, benzoic acid, 3,5-dihydroxybenzoic acid, and 5,5-thiodisalicylic acid); silver salts or complexes of thioenes (e.g., 3-(2-carboxy-ethyl)-4-hydroxymethyl-4-(thiazoline-2-thioene and 3-carboxymethyl-4-thiazoline-2-thioene); silver complexes or salts of nitrogenous acids such as imidazoles, pyrazoles, urazoles, 1,2,4-thiazoles, 1H-tetrazoles, 3-amino-5-benzyl-thio-1,2,4-triazoles, and benzotriazoles; silver salts of saccharin and 5-chlorosalicylaldoxime; and silver salts of mercaptides. The preferred silver source is silver behenate. The reducible silver source is preferably used in an amount of up to 3 g/m2, more preferably up to 2 g/m2 of silver. The lower limit is usually 0.1 g/m2, though not critical.
An antifoggant may be contained in the thermographic recording element according to the invention. The most effective antifoggant was mercury ion. Use of a mercury compound as the antifoggant in photosensitive material is disclosed, for example, in U.S. Pat. No. 3,589,903. Mercury compounds, however, are undesirable from the ecological aspect. Preferred in this regard are non-mercury antifoggants as disclosed, for example, in U.S. Pat. Nos. 4,546,075 and 4,452,885 and JP-A 57234/1984.
Especially preferred non-mercury antifoggants are compounds as disclosed in U.S. Pat. Nos. 3,874,946 and 4,756,999 and heterocyclic compounds having at least one substituent represented by xe2x80x94C(X1) (X2) (X3) wherein X1 and X2 are halogen atoms such as F, Cl, Br, and I, and X3 is hydrogen or halogen. Preferred examples of the antifoggant are shown below. 
More preferred antifoggants are disclosed in U.S. Pat. No. 5,028,523, British Patent Application Nos. 92221383.4, 9300147.7 and 9311790.1.
In the photothermographic material according to the preferred embodiment of the invention, there may be used sensitizing dyes as disclosed in JP-A 159841/1988, 140335/1985, 231437/1988, 259651/1988, 304242/1988, and 15245/1988, U.S. Pat. Nos. 4,639,414, 4,740,455, 4,741,966, 4,751,175, and 4,835,096.
Useful sensitizing dyes which can be used herein are described in Research Disclosure, Item 17643 IV-A (December 1978, page 23), ibid., Item 1831 X (August 1978, page 437) and the references cited therein.
It is advantageous to select a sensitizing dye having appropriate spectral sensitivity to the spectral properties of a particular light source of various scanners. Exemplary sensitizing dyes include (A) simple merocyanines as described in JP-A 162247/1985 and 48653/1990, U.S. Pat. No. 2,161,331, W. German Patent No. 936,071, and Japanese Patent Application No. 189532/1991 for argon laser light sources; (B) tri-nucleus cyanine dyes as described in JP-A 62425/1975, 18726/1979 and 102229/1984 and merocyanines as described in Japanese Patent Application No. 103272/1994 for Hexe2x80x94Ne laser light sources; (C) thiacarbocyanines as described in JP-B 42172/1973, 9609/1976, 39818/1980, JP-A 284343/1987 and 105135/1990 for LED light sources and red semiconductor laser light sources; and (D) tricarbocyanines as described in JP-A 191032/1984 and 80841/1985 and 4-quinoline nucleus-containing dicarbocyanines as described in JP-A 192242/1984 and 67242/1991 (as represented by formulae (IIIa) and (IIIb) therein) for infrared semiconductor laser light sources.
These sensitizing dyes may be used alone or in admixture of two or more. A combination of sensitizing dyes is often used for the purpose of supersensitization. In addition to the sensitizing dye, the emulsion may contain a dye which itself has no spectral sensitization function or a compound which does not substantially absorb visible light, but is capable of supersensitization.
For exposure of the photothermographic material of the invention, an Ar laser (488 nm), Hexe2x80x94Ne laser (633 nm), red semiconductor laser (670 nm), and infrared semiconductor laser (780 nm and 830 nm) are preferably used.
A dyestuff-containing layer may be included as an anti-halation layer in the photothermographic material of the invention. For Ar laser, Hexe2x80x94Ne laser, and red semiconductor laser light sources, a dyestuff is preferably added so as to provide an absorbance of at least 0.3, more preferably at least 0.8 at an exposure wavelength in the range of 400 to 750 nm. For infrared semiconductor laser light sources, a dyestuff is preferably added so as to provide an absorbance of at least 0.3, more preferably at least 0.8 at an exposure wavelength in the range of 750 to 1500 nm. The dyestuffs may be used alone or in admixture of two or more. The dyestuff may be added to a dyestuff layer disposed on the same side as the photosensitive layer adjacent to the support or a dyestuff layer disposed on the support opposite to the photosensitive layer.
Various supports are used in the invention. Useful supports are paper, synthetic paper, synthetic resin-laminated paper (exemplary synthetic resins being polyethylene, polypropylene and polystyrene), plastic films (e.g., polyethylene terephthalate, polycarbonate, polyimide, nylon, and cellulose triacetate), metal sheets (e.g., aluminum, aluminum alloys, zinc, iron and copper), paper sheets and plastic films having such metals laminated or evaporated thereon.
When plastic film is passed through a thermographic processor, the film experiences dimensional shrinkage or expansion. When the thermographic recording element is intended for printing purposes, this dimensional shrinkage or expansion gives rise to a serious problem against precision multi-color printing. Therefore, the invention favors the use of a film experiencing a minimal dimensional change. Exemplary materials are styrene polymers having a syndiotactic structure and heat-treated polyethylene. Also useful are materials having a high glass transition temperature, for example, polyether ethyl ketone, poly-styrene, polysulfone, polyether sulfone, and polyarylate.